Schematic illustrations of micro-sized pores (structural defects) in (a) CrN- and (b) TiSiN-coated steels immersed in a corrosive solution. Micro-sized particles also exist in both types of coatings, in this case, sitting in the middle of pore regions. The oxide layer dominant on the surface of CrN coating shows a greater integrity than that of TiSiN, preventing the corrosive agents from penetrating through the coating. Credit: JACerS; Wiley.
One of the things I like about ceramic materials is the way they work in concert with other materials. Coatings are a good example. A paper published in the September issue of the Journal of the American Ceramic Society takes a look at the role of ceramic coatings in the ongoing battle against corrosion, this time with respect to tool steels. The work was done by a multi-university team in Australia.
Tool steels might more accurately be called “tooling” steels—they are the steels formed into most of the cutting tools used for machining operations. There are many tool steel alloys, each designed for specific applications, such as high-speed machining, shock-inducing situations (e.g., chisels), etc. Generally, tool steels need to be tough, thermally stable in the working temperature range and chemically inert. Being chemically inert is important because much machining is done with a lubricant, and when a tool is not being used, it is exposed to the ambient atmosphere of a toolbox.
Corrosion, as we know, is a very expensive problem. A 2008 article in Science cited a government report saying that corrosion of steel components costs about three percent of the annual global GDP. According to World Bank statistics, the 2008 GDP was $61.2 trillion; the 2011 GDP was just under $70 trillion. So, the losses to corrosion could be estimated to have increased by about $300 billion in the three-year period between 2008 and 2011. It is not clear whether corrosion correlates proportionately to GDP or not, but clearly, getting control of corrosion should be a global priority.
Chromium nitride (CrN) is a typical corrosion protection coating applied to machine tooling. The grains of CrN coatings have an elongated morphology that shear during mechanical loading and thereby prevent the formation of cracks in the coating. The paper reports that recently there has been interest in titanium-silicon nitride nanocomposite (TiN nanoparticles in an amorphous SiNx matrix) coatings because they are extremely hard. However, hardness comes at the price of brittleness.
The authors note that it is not unusual for sputtered ceramic coatings to have defects like pores or microparticles, which create opportunities for corrosive agents to breach the coating and attack the metal. The literature also reports that other researchers have observed that a passive oxide coating forms on the surface of both CrN coatings and TiSiN coatings. In the paper, the authors say the oxide layer is “critical to amend the structural defects and restore the ability of the coated metals against corrosion.” But, what is not clear is how. Also, the authors say, not much work has been done to determine what effect damage to the coating during machining has on the corrosion resistance of the tool. After all, mechanical damage is unavoidable during machining operations.
Thus, the goals of this research were to understand the nature and role of the oxide layer and the effect of mechanical damage on the corrosion resistance efficacy of the coatings.
CrN and TiSiN coatings were sputtered onto AISI M42 tool steel substrates, and nanoindentation was used to apply simulated, controlled mechanical damage to the coatings. Corrosion tests and extensive microstructural characterization were done, and shear stress effects were evaluated by finite element analysis.
Schematic representation of (a) deformed CrN and (b) damaged TiSiN coatings on steels immersed in corrosive solutions. Credit: JACerS; Wiley.
The results showed that both the oxide layer and the mechanical damage deformation mode “played significant roles in controlling the corrosion damage of ceramic coatings on steels.”
Sputtered coatings often have defects like pinholes and inclusions, and the oxide layer isolates these defects from corrosive agents. They found, however, that the chromium oxide coating that forms on the CrN “shows a greater integrity than [the oxide layer] of the TiSiN.”
Similarly, when the CrN coatings responded better to shear stress than the TiSiN coatings. The paper says, “shear deformation, enabled by the columnar structure in the CrN coatings, reduced the stress concentration and averted the formation of open cracks during mechanical loading, imparting to the coated steels a greater corrosion resistance.”
In contrast, open cracks formed in the TiSiN coatings from the concentration of shear stresses. The cracks “acted as the pathway of corrosive agents, causing severe damage in the steel substrates.”
The paper is “Corrosion- and Damage-Resistant Nitride Coatings for Steel,” M.S. Ahmed, et al., JACerS (doi: 10.1111/j.1551-2916.2012.05328.x)